copy JNI from AAR files to android_app APK

JNI libs present in AAR files are ignored in the build. If those
libraries are necessary, they must be manually extracted and a
prebuilt_cc_library module must be created for them. Instead, we can
extract these libraries and copy them to an output APK as part of the build.

Bug: 228848794
Test: created android_app that depends on an android_library_import with
  JNI embedded, and verified that .so libs are present in APK
Test: created android_app that depends on an android_library which
  depends on android_library_import with JNI embedded, and verified
  that .so libs are present in APK
Test: verified that multiple .so's from a single architecture are copied
  to APK
Test: verified that the absence of a jni/<arch> folder or files in the
  folder will cause the Ninja action to error
Change-Id: I242a862d7bd881f7a4a8c23493924d8fe441ea25